1. Field of the Invention
The present invention relates to an encoding and decoding method and device for compression encoding, transmitting and decoding of information of an image signal or audio signal.
2. Description of the Prior Art
In the field of image or voice communication, there has so far been performed compression of signals or highly efficient encoding of data in order to efficiently utilize the limited line capacity. One practical example is an encoding and decoding device for used in TV conference or TV telephone.
"A Fractal Based Approach To Image Compression", ICASSP, 86, 11A.3 is known to the present inventors as a reference closely related to the present invention. This reference is to compress image data using the fractal concept. The basic concept is in that "yard stick" of certain fixed length is used as a unit to cover a signal, and a horizontal distance of the cross point at which "yard stick" intersects the signal, i.e., sampling interval and a positive or negative sign bit respectively indicative of increase or reduction in an amplitude value are both transmitted for restoration of the signal at the decoding side. The practically implementing method employs "trigger function" in place of "yard stick", the trigger function being compared with the signal to determine a sampling interval. This makes the sampling interval wider for a signal including many flat parts, so that the signal may be compressed to a large extent.
However, because "trigger function" is a unique function for all of successive data samples, the above method cannot follow a multi-value graded image including steeply changing parts, resulting in a problem that the image may be blurry in its edge. Another problem is in that since representative values used in the decoding step are given by those threshold values which were used in the encoding step, the decoded values are always less than the amplitude values of the original signal, and hence the mean square error is increased. It is to be noted that the above reference teaches only the method, and does not propose a practical device.